In the process of papermaking, many stages are required to transform headbox stock into paper. The initial stage is the deposition of the headbox stock onto paper machine clothing or fabric. Upon deposition, the white water forming a part of the stock, flows through the interstices of the fabric, leaving a mixture of water and fiber thereon. The fabric then supports the mixture, leading it through several dewatering stages such that only a fibrous web or matt is left thereon.
One of the stages of dewatering takes place in the nip press section of the papermaking process. In the nip press section, two or more cooperating rolls press the fibrous web as it travels on the fabric between the rolls. The rolls, in exerting a great force on the fabric, cause the web traveling thereon to become flattened, thereby achieving a damp fibrous matt. The damp matt is then led through several vacuum and dewatering stages.
The amount of pressure applied to the web during the nip press stage is important in achieving uniform sheet characteristics. Variations in nip pressure can affect the sheet moisture content and sheet properties. Excessive pressure can cause crushing of the fibers as well as holes in the resulting paper product. Conventional methods to solve this problem have been unsuccessful, and as such this problem persists in the nip press stage, often resulting in paper of poor quality having uneven surface characteristics.
Roll deflection, commonly due to sag or nip loading, has been a source of uneven pressure distribution. To compensate for such deflection, rolls have been developed which monitor and alter the roll crown. Such rolls usually have a floating shell which surrounds a stationary core. Underneath the floating shell are pressure regulators which detect pressure differentials and provide increased pressure to the floating shell when necessary.
One such roll is described in U.S. Pat. No. 4,509,237. This roll has position sensors to determine an uneven disposition of the roll shell. The signals from the sensors activate support or pressure elements underneath the roll shell, thereby equalizing any uneven positioning that may exist due to pressure variations. The pressure elements comprise conventional hydrostatic support bearings which are supplied by a pressurized oil infeed line. A similar roll is disclosed in U.S. Pat. No. 4,729,153. This controlled deflection roll further has sensors for regulating roll surface temperature in a narrow band across the roll face. Other controlled deflection rolls such as the one described in U.S. Pat. No. 4,233,011 rely on the thermal expansion properties of the roll material, to achieve proper roll flexure. Such deflection compensated rolls are effective in varying the crown. Thus, such rolls can operate as effectively at a loading of 100 pounds per inch as at 500 pounds per inch, whereas rolls without such capabilities can only operate correctly at a single specific loading.
Although the prior art has addressed the problem of measuring roll deflection, the prior art is silent as to methods of measuring the loading across the roll face while the roll is in operation. Loading is the force which the roll applies in a press nip, to a fibrous web. As stated above, often the amount of pressure is unevenly applied. For example, if roll loading is set to 200 pounds per inch, it may actually be 300 pounds per inch at the edges and 100 pounds per inch at the center.
Conventional methods of determining the presence of such discrepancies in applied pressure requires stopping the roll and placing a long piece of carbon paper, foil, or pressure sensitive film in the nip. This procedure is known as taking a nip impression. While this procedure is useful, it cannot be used while the nip press is in operation. Furthermore, such methods are not reusable as they measure only a single event such as the highest pressure or contact width. Additionally, such readings, to be useful must be repeatedly obtained, and averaged, a process which results in increased down time for unloading and reloading of the paper. Lastly, temperature and other related changes which would affect the uniformity of nip pressure, cannot be taken into account.
The roll described in U.S. Pat. No. 4,898,012 has attempted to address this problem by incorporating sensors on the roll to determine the gauge profile of a press nip. However, there are a number of problems inherent to this roll. The construction of this roll requires a stationary center beam, and as such, would not be adapted to all types of rolls, but only rolls having a floating roll shell, such as controlled deflection rolls. Therefore, the approach could not be implemented on existing non-controlled deflection rolls. The technique would require significant calibration since the measurements are based upon the deflection of the floating shell inside diameter and not the actual nip load.
Conventional roll-systems in the prior art have failed to provide measurements of pressure variations therealong while the roll is rotating in a press nip. The instant invention measures such variations and provides the operator with instantaneous knowledge of such pressure variations, thereby enabling the operator to diagnose irregularities in the pressure applied to the web, and initiate corrective measures without delay.